-----------------------------------------------AC code-----------------------------------------------

#include<algorithm>
#include<cstdio>
#include<stack>

using namespace std;

const int MAXN = 100005;
const int MAXM = 500005;

int n, m, u[MAXM], v[MAXM];
int sum_ind, sum_outd;

struct Edge{
    int nxt, to;
}edge[MAXM], e[MAXM];

int head[MAXN], edge_num;
void add_edge(int from, int to){
    edge[++edge_num].nxt = head[from];
    edge[edge_num].to = to;
    head[from] = edge_num;
}

int hd[MAXN], e_n, ind[MAXN], outd[MAXN];
void a_e(int from, int to){
    e[++e_n].nxt = hd[from];
    e[e_n].to = to;
    hd[from] = e_n;
    ind[to]++;
    outd[from]++;
}

int dfn[MAXN], low[MAXN], scc, belong[MAXN], dex;
stack<int> s;
bool ins[MAXN];
void tarjan(int x){
    dfn[x] = low[x] = ++dex;
    s.push(x);
    ins[x] = 1;
    for(int i = head[x]; i; i = edge[i].nxt){
        if(!dfn[edge[i].to]){
            tarjan(edge[i].to);
            dfn[x] = min(dfn[edge[i].to], dfn[x]);
        }
        else
            if(ins[edge[i].to])
                dfn[x] = min(dfn[x], low[edge[i].to]);
    }
    if(dfn[x] == low[x]){
        scc++;
        while(1){
            int cur = s.top(); s.pop(); ins[cur] = 0;
            belong[cur] = scc;
            if(cur == x)    break;
        }
    }
}

int main(){
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= m; i++){
        scanf("%d%d", &u[i], &v[i]);
        add_edge(u[i], v[i]);
    }
    for(int i = 1; i <= n; i++)
        if(!dfn[i])
            tarjan(i);
    for(int i = 1; i <= m; i++)
        if(belong[u[i]] != belong[v[i]])
            a_e(belong[u[i]], belong[v[i]]);
    if(scc == 1){
        putchar('0');
        return 0;
    }
    for(int i = 1; i <= scc; i++){
        if(ind[i] == 0) sum_ind++;
        if(outd[i] == 0)    sum_outd++;
    }
    printf("%d", max(sum_ind, sum_outd));
    return 0;
}